The present invention is related to a feed method and apparatus for smelting and melting furnaces. More specifically, an additive feed apparatus is disclosed for tuyere-equipped, vertical-shaft furnaces, which apparatus utilizes a gravity feed method to obviate powered entrainment and transmission means, such as pneumatic injection apparatus. The additive-feed apparatus provides for the direct charging and utilization of various materials in vertical-shaft furnaces, such as blast furnaces and cupolas, which various materials are not usually utilized for direct introduction with the top-charged burden materials.
In both of the above-noted furnace types, the raw or burden materials are generally charged through the top of the furnace. In a blast furnace, the iron ore or iron-bearing charge material may consist of any of the forms or oxidation states of iron, which are reduced in a reducing atmosphere at elevated. temperatures. Although it is known that blast furnaces have been run without a pressurized top, modern furnace practices utilize pressurized furnaces with feed hoppers having a dual-bell system to maintain the internal furnace pressure during charge additions.
The chemical and thermodynamic reactions in the vertical-shaft furnace require a combination of materials in the burden including coke, iron-bearing materials and limestone. The coke is a multifaceted addition to this burden. It reacts with the oxygen in the blast air blown into the furnace to burn and provide the reaction heat, which blast air may be enriched with oxygen or other gasses. Coke combustion products include carbon monoxide, which acts to reduce the iron oxides to elemental iron particularly in the upper regions of the furnace. The hot gasses evolved during carbon combustion at the tuyere region preheat the burden materials at the upper reaches of the furnace, gasses at least partially dry and prereduce the other raw materials. The coke charge also has a mechanical function in the furnace reaction, as it must be able to sustain the overlying burden weight without being crushed, which preserves a path for ready flow of the gasses through the burden above the hearth.
The ores and other iron-bearing charge materials are not pure iron oxide but rather are frequently mineral bearing materials laden with extraneous or gangue components. Therefore, lime usually in the form of limestone is added to the burden to flux the molten iron and to generate a slag. This slag also helps to purge the ash, sulfur and residue or byproduct materials from combustion of the coke. The limestone addition requires a determinable amount of coke to calcine, melt and raise the temperature of the limestone addition, as this is basically an endothermic reaction.
The cupola is a vertically oriented, cylindrical, shaft-type furnace generally having a steel shell and it is somewhat similar in appearance to a blast furnace, but not necessarily analogous in operation. The cupola is the most prevalent furnace utilized in iron foundries for the production of various types of cast iron and may be run as a semi-batch or continuous type operation. The cupola charge or burden materials differ from the blast furnace raw materials as it utilizes steel scrap, iron scrap and pig-iron rather than iron ore. Also, a cupola has tapholes and runners for the slag and molten metal, but generally does not operate with a pressurized feed hopper like a blast furnace. All of these physical characteristics bear evidence to the similarities of these furnaces.
The cupola blast air system is not unlike that of a blast furnace, as it introduces combustion air for the coke into the furnace through tuyeres. The blast air is introduced to the cupola volume at a lower pressure, such as in the range of about 10 to 80 ounces per square inch above atmosphere, through the tuyeres. The coke is burned and the metallic charge is melted. Carbon control in the as-tapped molten metal is broadly a function of the amount of coke charged to the furnace and the carbon present in the charged iron and steel scrap.
In the processing of materials for charging to a cupola, the raw material additions are frequently sized by screening or other means to provide a more uniform material component and to avoid the introduction of small sized additions, which may oxidize rapidly outside the melting zone or be entrained in the gaseous emissions discharge for entrapment in a baghouse. As a specific example, coke may be screened to minimize addition to the furnace of materials which are less than about one and three-quarter inches in diameter. The screened discards are set aside for temporary storage prior to resale to a vendor, but are generally not utilized in the cupola furnace because of their relatively small size.
Metallurgical coke is an expensive commodity and the losses of the screened material may be as high as ten or twenty percent. Further, the screened coke discard material is susceptible to moisture pickup from outside storage, and both the undersize condition and moisture content are regarded as detrimental to a furnace operation. The introduction of moisture to a cupola results in heat losses, as it requires heat to evaporate the water, which consequently requires the addition of more coke and, therefore, the entrained sulfur and ash to the furnace. Thus, it is apparent that dry coke additions are generally easier on the furnace operator, give more consistent results and are, consequently, more desirable.
Historically the cupola operator has had to find supplemental uses for the screened coke discards or frequently has had to find a secondary vendor for these materials. As an example, metallurgical coke may cost $180 per ton but the undersized discards are only resalable for about $25 per ton, which results in lost material, handling, storage, recovery and replacement costs. Therefore, furnace operators have continuously tried to find methods and apparatus to utilize these screened and discarded materials. One known use of these discarded material additions is in the production of iron sinter in sintering plants of steel mills, which use discarded iron, lime and coke fines to produce a material acceptable for charging to a blast furnace. Unfortunately, this is an expensive operation, which was used to consume all the chemically valuable raw materials that were physically unchargeable to furnaces. Many of these sintering plants have been abandoned as they are difficult to run and maintain, and the cost of handling the air emissions from these plants may be disproportionate to, the gains from their operations.
Indicative of various methods devised to utilize coke and coal are a coal-oil slurry method disclosed in U.S. Pat. No. 4,030,894. Other methods utilize finely pulverized coke and coal additions, which may be introduced in a carrier gas stream for entrainment in the hot-blast gasses. However, any of these noted methods require comminuting the coke or coal to a size such as 100 mesh by down or similar size. In addition, the material must be dried prior to furnace introduction, the moisture content must be carefully controlled, or such moisture must be otherwise accommodated. The materials are usually introduced through the furnace tuyeres by a secondary, cold-air, gas carrier. Again, as in a sintering operation, there is a secondary handling and processing of the addition prior to its introduction to the furnace. Another impediment to the utilization of these materials in the furnace operations is the education of the operators to accommodate their introduction and the consequent effects upon both the heat and mass balance, the temperature variations and resultant chemical changes of both the slag and molten metal. Consequently, there has been a reticence to utilize these secondary materials as furnace additions because of the added costs and disruptions to presently accepted operating practices.